The present invention relates to the noise reduction of information signals. In particular, the invention relates to an apparatus and method for providing compression and expansion of information signals prior to and following transmission or recording.
Many methods for compressing and expanding of information signals including audio and video signals have been proposed and implemented since the late 1960s. These methods are to effect an increase in dynamic range and, hence, a reduction of noise for signals which are transmitted or recorded using a medium that has a limited dynamic range.
U.S. Pat. Nos. 3,846,719 and 3,903,485 disclose a process which forms the basis of a commercial system known as A-type noise reduction which divides the frequency spectrum into 4 bands with crossover points at 80 Hz, 3 kHz, and 9 kHz. The process provides a maximum noise reduction of about 10 dB for the three lower bands and about 15 dB for the uppermost band. In the presence of a dominant signal this process functions best with a frequency that falls in the middle region of one of the bands. In such a case this will cause the band to compress whilst the other three bands are largely unaffected. Thus the noise reduction action is maintained over a large portion of the spectrum. One drawback of this system is that if a dominant signal has a frequency in the region of one of the crossover points, then both of the bands which are separated by that crossover frequency are compressed and hence there is a loss of noise reduction over two entire bands. This is particularly a problem in the mid frequency region where the bandwidth of the mid band (Band 2) is very broad, being from 80 Hz to 3 kHz. Thus, a dominant signal with a frequency in the region of 3 kHz results in compression of the mid upper bands (Bands 2 and 3) which collectively cover from 80 Hz to 9 kHz, (i.e. the majority of the audio frequency spectrum) so the loss of noise reduction action is very considerable.
U.S. Pat. No. Re 28,426 discloses a process which forms the basis of a commercial system known as B-Type noise reduction which uses a variable filter in combination with a fixed high pass filter to achieve a degree of noise reduction in the high frequency part of the spectrum. This system solves some of the problems of the Dolby A-type system but introduces new problems in that a dominant high frequency signal component will cause the sliding filter to slide upwards to a point above the frequency of the dominant component. This results in a substantial loss of noise reduction below the frequency of the dominant component.
U.S. Pat. No. 4,490,691, discloses a process which forms the basis of a commercial system known as C-type noise reduction and part of the basis of a later system known commercially as Spectral Recording. The C-type process was devised to improve the performance of the B-type system by the use of multiple sliding band filters in series and various frequency pre-emphasis and de-emphasis techniques. However, like the B-Type noise reduction system, this process still suffers from a significant loss of noise reduction in the frequency region below a high frequency high level dominant signal component.
U.S. Pat. No. 4,736,433 discloses a process which along with U.S. Pat. No. 4,490,691, forms part of the basis of a commercial system known as Spectral Recording which was devised to address the combined limitations of the A-type, B-type and C-type systems using a process called xe2x80x9caction substitutionxe2x80x9d which combines fixed and sliding band compressors. The Spectral Recording process divides the audio spectrum into just two bands with a crossover at 800 Hz with a maximum noise reduction of about 16 dB for the low frequency band and about 24 dB for the high frequency band. The process employs a fixed band compressor and a sliding band compressor in each of the upper and lower frequency bands. The fixed band compressor covers the full bandwidth of the frequency band and the sliding band compressor slides upward in the case of the high frequency band and downward in the case of the low frequency band with increasing signal level. This system maintains a high level of noise reduction in the upper and lower portions of the audio spectrum particularly in the presence of dominant signals with frequencies in the middle region of the audio spectrum because the sliding filters slide outwards from the middle of the spectrum, thus overcoming the main drawback of the A-type system. However, if a dominant signal has a frequency at the upper or lower ends of the spectrum there is still a potential for a significant loss of noise reduction in the upper middle region in the case of a high frequency dominant signal or in the lower middle region in the case of a low frequency dominant signal.
The present invention overcomes the above mentioned drawbacks of all prior art noise reduction processes by creating the divisions between the frequency bands by the use of variable crossover circuits, the variable crossover frequencies being controlled by the spectral and level content of the input signals.
An apparatus and method for compressing and expanding an information signal for use in noise reduction which includes an input terminal to receive the information signal, a first frequency crossover circuit coupled to the input terminal and configured to divide the information signal into a first and second frequency band. A second frequency crossover circuit coupled with the first crossover circuit configured to receive at least a portion of the information signal within the first frequency band and to further divide the information signal into a third and fourth frequency band. A third frequency crossover circuit coupled with the first crossover circuit configured to receive at least a portion of the information signal within the second frequency band and to divide the information signal into a fifth and sixth frequency band.
In one embodiment the frequency crossover circuits are variable frequency crossover circuits such that the frequency bands are separated by a variable crossover frequency and each frequency crossover circuit includes feedback control.
In one embodiment each frequency crossover circuit includes feedback control which includes weighting circuits to provide greater sensitivity to frequency of the information signal.
In one embodiment, the second and third frequency crossover circuits include a one compressor circuit for each frequency band and configured to compress the band when a dominant signal is present in the information signal which falls within a predefined frequency range.
One advantage of the invention is that it provides a compressor and expander implemented through a relatively simple circuit design.
Another advantage is that the invention is highly adaptive to constantly varying real world signals.
Another advantage is that the invention limits the amount of compression of the information signal by providing a plurality of bands separated by a plurality of variable crossover frequencies.
Another advantage is the invention minimizes mid frequency modulation effects by limiting the shifting of the variable crossover frequencies.
Yet another advantage is that the invention limits the bandwidth which is compressed when a dominant signal is present.
Another advantage is that the invention limits the compression of frequency bands which do not include a dominant signal, by shifting the crossover frequencies.